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Uh Hello everyone and welcome to this med BRS One, a tutorial on embryo, fetal development and disorders. So today we're gonna talk a bit about um how embryology is researched and as well as talking about the development of a new human, we're also gonna talk about disorders of early development and pregnancy. So, since embryology is a research intensive field and conducted across many species, there are some standardized measures of time and development ways of measuring the age of an embryo, include fertilization and gestational ages. So, fertilization age also known as conceptual age is the time elapsed since fertilization of the site or the oven er and gestational age is the time elapsed since the woman's last menstrual period. And it can be calculated by taking the fertilization age and adding two weeks. The Carnegie staging is a system of 23 stages common to all mammals that allow for interspecies comparison. But I'll go more into this in the next slide. On top of these, the pregnancy can be divided into three stages. The embryogenic stage, which is the stage of embryogenesis and the beginning of cellular differentiation. The embryonic stage, which is when the body plan is established and the fetal stage, which is when the body plan has been established and the fetus grows so more into the Carnegie staging. So, as I said, it's a standardized series of 23 stages that covers the embryogenic and embryonic stages. It's based on the physical characteristics and appearance of the developing vertebrate rather than the age. So how does it all start with fertilization? Of course, fertilization is the joining of male and female pronuclei usually in the fallopian tubes of the woman. What's important to note is that during this process, the ovum reinforces its zona pellucida to prevent polyspermy and contribute to embryogenesis. So what takes place in the 1st 16 days? Well, after fertilization, the resultant cell is a zygote and due to the spatial limitations of a zona pellucida, the zygote undergoes a special form of mitosis called cleavage, which is when the cell simply halves with no previous growth. The cleavage stage is also when the embryo passes its first milestone up until day 2 to 3. The zygote derived cells also called blastomere are reliant on a reserve of Mrna proteins and other nutrients built up in the developing oven. This will not however last forever. The blastomas must begin engaging their own cellular machinery to produce proteins in a turn called the maternal to zygotic transition. Now, once the embryo is past 16 cells, it is known as a morular in the morular. There are enough cells to form two spatially distinct populations of cells. Those touching the zona paluster become wedge shaped, polarized and form tight gap junctions in between. This is called compaction and allows the outer cells to control the environment of the inner cells. So those outer cells have now differentiated into trophoblast cells forming the ectoderm. For now, the role of these cells is to pump sodium ions into the inner cell environment. Water then follows B osmosis creating a cavity with an inner cell mass of pluripotent embryonic stem cells. At one end, the cavity is called the blastose. And the entire structure is now referred to as a blastocyst. And at this point, the zona pellucida has served its purpose. The blasticus needs to escape in order to allow for implantation and it hatches from the zona pellucida by a combination of enzymatic digestion and cellular contractions. When the newly free blastocyst contacts the endometrium, the extraembryonic trophectoderm starts to further differentiate those uh cells touching the endometrium fuse becoming a single multinuclear syncytiotrophoblast. The rest of the trophoblasts becomes cytotrophoblast which act as a reserve of cells to fuse with the syncytiotrophoblasts as it grows. And the role of this initio trophoblast is to invade the endometrium, breaking down the tissue and drawing nutrients for the embryo. At the same time, the embryo itself is differentiating more those inner cells bordering the blastocoel become hypoblasts which will form the yolk sac. The rest of the inner cell mass becomes epiblast. Now by day 12, this epiblast forms a new amniotic cavity. Some cells come away from the trophectoderm leaving behind a membrane called the amnion, which is one of the fetal membranes. The embryo now contains two cavities with a disc in between. This is called the bi lamina disc named. So for the two different cell types, the epiblast and hypoblast and finally, the embryo is fully submerged in the endometrium. We say it is implanted at this point. The syncytiotrophoblast is producing beta HCG to maintain the endometrium and is extensively searching for maternal capillaries and uterine glands to facilitate placentation. The cytotrophoblast are also doing the same through extension of finger like chorionic vili which we'll look at more later on. But for now, we'll leave the extraembryonic structures and focus just on the embryo. Carnegie stage six is the formation of the yolk sac which almost fully replaces the blast seal in an effort to aid in nutrition and gas exchange. Its formation involves expansion of the hypoblast around the blastocoel forming the exoloma membrane. And finally, embryogenesis ends with what may be the most important event event in anyone's development. The process of gastrulation is quite complex but gives rise to three main embryonic tissue lineages, ectoderm, mesoderm and endoderm as well as establishing the cranial caudal and left right axis. But how does it happen? Well, first, a primitive streak forms from the caudal end of the epiblast. This streak extends towards the cranial end widening and deepening into the primitive node. And pit epiblast cells then migrate towards this primitive streak invaginated through and flooding the remaining blastose between hypoblast and epiblast. These migratory epiblast cells seed the hypoblast replacing it and forming the definitive endoderm. Once this hypoblast is replaced, the migrant cells fill the space becoming the mesoderm migration then stops leaving behind a trilaminar disc of endoderm, mesoderm and ectoderm. This trilaminar disc is now ready to start forming an organism as I alluded to the three germ layers give rise to all adult tissues. The endoderm gives rise to the innermost organs. For example, the gi tract gi accessory organs such as the liver and pancreas, the lungs and the thyroid. The mesoderm lineage miso meaning middle includes blood cells, muscles, perirenal organs such as kidneys and the gonads and skeletal tissues such as the bone and cartilage. Finally, the ectoderm gives rise to more superficial tissues such as nervous tissue, skin and tooth enamel. So now let's look at some example, questions. So we've got two single best answer questions. From what point is gestation.